This invention relates to variable magnification or zoom lenses, and more particularly to a zoom lens for use between fixed conjugate planes, as for example in a photocopying environment.
A zoom or varifocal lens is one in which the focal length can be continuously varied by moving one or more of the lens elements along the axis of the lens, the image being maintained in focus by either mechanical or optical means. The technology behind variable focal length lenses is relatively new in the long history of optics, having had most of its development since World War II. Many zoom lenses are presently known and are used in many different applications, including television, photography, and photocopying to which the present invention is primarily directed. In these prior art lenses, many different forms of interrelated lens movements are shown. These include symmetrical and asymmetrical lenses and lens movements, and a wide variety of interrelations of lens movement. U.S. Pat. No. 3,687,522 to Lynch and Carlough is considered representative of the art and the particular application to which the present invention is directed. Therein is disclosed a zoom lens in which the lens elements and their zoom movement are all symmetrical. In this 6-element lens, the end crown elements and their adjacent flint elements move in opposite directions, thereby changing the crown/flint spacings and the flint/meniscus spacings. Field curvature introduced by changing the separation of crown and flint elements is compensated for by the changed separation of the flint and meniscus elements. These movement of the crown and flint elements are different in extent because the lens is more sensitive to variation of axial separation between flint and meniscus elements than between crown and flint elements. Further, since the fixed part of the lens is the inner assembly of menisci, the mounting is relatively complex and costly.
It is an object of this invention to provide a variable magnification lens in which the outer elements remain fixed with respect to the lens.
Another object of this invention is to provide such a variable magnification lens in which the image field is kept substantially flat throughout the magnification range.
Another object of this invention is to provide such a variable magnification lens in which both the lens elements and their movement are symmetrical with respect to the lens center.
Other objects, advantages, and features of this invention will become apparent from the following more detailed description of an exemplary embodiment given in connection with the accompanying drawing.